Method and device for establishing the network topology of a bus system

ABSTRACT

To determine the network topology of a bus system having a number of bus users positioned on bus segments of a bus line interconnected by at least one diagnosis repeater, an initiating measuring telegram is first sent to each bus user. This telegram is answered by the diagnosis repeater with a data telegram having a segment code and by other bus users with a response telegram. The diagnosis repeater then sends a measuring signal to each responding bus user, which reflects the signal. The distance of the responding bus user from the diagnosis repeater is determined from the time interval between the sending of the measuring signal and the arrival of the reflection signal.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and hereby claims priority to GermanApplication No. 100 48 745.9 filed on Sep. 29, 2000, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The invention relates to a method for determining the networktopology of a bus system having a number of bus subscribers, which arearranged on mutually connected bus segments of a bus line. It alsorelates to an apparatus for carrying out the method.

[0004] 2. Description of the Related Art

[0005] A bus system or field bus within a complex network having anumber of bus segments has a large number of bus subscribers in the formof field appliances, such as sensors, actuators, programmable logiccontrollers and control stations. By way of example, the so-calledPROFIBUS DP network normally has up to 127 bus subscribers, to whichsubscriber addresses from 0 to 126 may be allocated. In this case, up to32 network components can be connected as bus subscribers to one bussegment, as a section of a bus line.

[0006] The line length of an individual bus segment is limited,depending on the so-called baud rate. This indicates the data rate whichcan be transmitted over that bus segment. With a normal data rate of 12M baud=12·10⁶ bits/s, it follows that one bit is equal to a length of 83ns. Within the network, the individual bus segments are connected byline drivers or so-called repeaters, which are used to provide branchesin the field bus or network. In this case, the network topology of thebus system should be known, and is governed by the arrangement, that isto say by the sequence and the relative distance between the bussubscribers on the individual bus segments (bus segment topology), andby the connection of the bus segments to one another.

SUMMARY OF THE INVENTION

[0007] The invention is thus based on the object of specifying aparticularly suitable method for automatically determining the networktopology of even a complex bus system. Furthermore, an apparatus whichis particularly suitable for carrying out the method will be specified.

[0008] The invention is in this case based on the knowledge that, in thecase of a bus subscriber with a transmitter which can be switched off,the transmitter/receiver combination when the transmitter is deactivatedon the one hand has such a high impedance that it does not cause anyinterference with the transmission on the bus line, but on the otherhand has a low impedance when the transmitter is activated. In thiscase, a transmitting bus subscriber reduces its internal impedance frominfinity (∞) to about 40 Ω, which results in a local change in thecharacteristic impedance of the bus line of the respective bus segmentto which this transmitting bus subscriber is connected. If a measurementpulse or signal is now passed to the bus line during the transmission ofa bus subscriber, then this pulse or this signal is reflected from thetransmitting bus subscriber. This reflection can in turn be detectedusing the principle of reflection measurement. A method which operateson the principle of reflection measurement for localization of a shortcircuit or of a cable discontinuity in a two-wire bus system isdisclosed, for example, in DE 197 26 539 A1.

[0009] In order to carry out the method according to the invention,repeaters which have an additional function added to them are used inthe bus system and, in the following text, these are referred to asdiagnosis repeaters. In addition to the repeater function for connectionof individual bus segments, these have a diagnosis function which isbased on the principle of reflection measurement. If diagnosis repeaterssuch as these are used instead of repeaters when designing a networkwith a number of bus segments, in particular when designing a PROFIBUSDP network, then no additional network components are required eitherfor determining the topology or for line diagnosis, which can likewisebe carried out by the diagnosis repeaters.

[0010] First of all, for topology determination, a request signal in theform of an initiating measurement message to send a response signal ormessage is transmitted via the bus line to each bus subscriber. Duringthe transmission of the response message, a measurement signal is sentvia the bus line, and is reflected by the responding bus subscriber.

[0011] Since a repeater as well as the diagnosis repeater which is usedin its place is a passive bus subscriber (slave), it cannot cause anyother bus subscriber to transmit. This object is therefore carried outby a master, which already exists in a bus system such as this, or bysome other active bus subscriber, which then has an appropriateinitiator function (measurement initiator) added to it, so that, onceagain, no additional network component is required. The measurementinitiator thus requests the or each bus subscriber to transmit while thediagnosis repeater is transmitting a measurement pulse to the respondingbus subscriber, and the bus subscriber reflects this measurement pulse.

[0012] In this application of the principle of reflection measurement,the time interval between the transmission of the measurement signal andthe arrival of the reflection signal from the responding bus subscriberis used to determine its distance from the diagnosis repeater and henceits relative position within the bus segment. In this case, a furtherdiagnosis repeater which is provided on this bus segment can be designedsuch that it is modeled as a slave and has its own subscriber address.

[0013] The diagnosis function is also used for line diagnosis, that isto say to determine the fault location and the fault cause of linefaults which occur along the bus line, such as a short circuit, a cablediscontinuity or an incorrectly connected terminating impedance.Following the automatic determination of the network topology, the faultlocation can then be indicated relative to the existing bus subscribers,for example in the form of “fault between subscribers Tx and Ty”.

[0014] This application of the principle of reflection measurement makesuse of the knowledge that a line fault or a disturbance point results ina local change to the characteristic impedance along a bus line, thusreflecting a pulse or a signal. For example, the characteristicimpedance changes to a low impedance in the case of a short circuit, andto a high impedance in the case of a discontinuity. The distance to aline fault or to a disturbance point can be determined from the timeinterval between the transmission of a measurement pulse or of ameasurement signal and the arrival of the reflection. Furthermore,statements relating to the cause of the line fault can be derived fromthe polarity of the reflection.

[0015] In one advantageous refinement, the distance to the or to eachbus subscriber is stored by the diagnosis repeater in a topology table.The topology table in each diagnosis repeater which is provided in thebus system or network has a number of table fields, which are associatedwith the subscriber addresses of all the bus subscribers. In this case,in particular, a status field is provided for indicating theidentification of the bus subscriber. A subscriber-specific bus segmentfield is also provided, containing an entry for the bus segments whichare connected to that diagnosis repeater. Furthermore, a distance fieldis provided, in which the distance determined to the respective bussubscriber found is entered. The topology table also includes a typefield for identifying an adjacent diagnosis repeater or another bussubscriber. Furthermore, a repeater-specific bus segment field isprovided, which is used for identifying the respective bus segment towhich the determined adjacent diagnosis repeater is connected.

[0016] If a disturbance point or a line fault along the bus line isdetermined during a process for determining disturbance points or linediagnosis which is likewise based, in an expedient development, on theprinciple of reflection measurement, the topology table isadvantageously used to determine the bus subscribers between which thedisturbance point or line fault is located.

[0017] This method and this apparatus for automatic topologydetermination make it possible to find out or determine the distancebetween each bus subscriber and a defined bus subscriber in the same bussegment, as well as their connection to one another, in a simple andreliable manner. In particular and in addition, in the event of atopology change, that is to say a change and/or an upgrade to theconfiguration of bus subscribers or bus segments, it is reliablypossible to exactly determine the sequence of and the relative distancebetween the bus subscribers within a bus segment, and their arrangementwithin the network, with particularly little effort.

[0018] The method according to the invention and the apparatus accordingto the invention are thus particularly suitable for field bus systems,in particular for the so-called PROFIBUS DP network.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] These and other objects and advantages of the present inventionwill become more apparent and more readily appreciated from thefollowing description of the preferred embodiments, taken in conjunctionwith the accompanying drawings of which:

[0020]FIG. 1 is a block diagram of a simple bus system with ameasurement initiator and a diagnosis repeater, which connects two bussegments, for topology determination,

[0021]FIG. 2 is a topology table which is stored in the diagnosisrepeater shown in FIG. 1,

[0022]FIG. 3 is a block diagram of a complex bus system with a number ofdiagnosis repeaters,

[0023]FIG. 4 is a topology table of a diagnosis repeater in the bussystem shown in FIG. 3,

[0024]FIG. 5 is a data structure of a segment identifier for a datamessage, and

[0025]FIG. 6 is a block diagram with data structures of data messageswhich are sent on a segment-specific basis by a diagnosis repeater fortopology identification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings, wherein like reference symbols refer to likeelements throughout.

[0027]FIG. 1 shows a detail of a relatively complex bus system 1, inparticular of a PROFIBUS DP network. In the case of a PROFIBUS DPnetwork, which may have a large number of bus segments Bn, a total of127 bus subscribers with the subscriber addresses T0 to T126 areprovided within the bus system 1. One bus segment Bn is in this case apiece or section of line of a bus line 2 to which up to 32 networkcomponents or subscribers Tn may be connected. The line length of therespective bus segment Bn is in this case limited—as a function of thebaud rate used for data transmission.

[0028] As is indicated in FIG. 1 and is illustrated in FIG. 3,individual bus segments Bn are connected by line drivers or lineamplifiers—so-called repeaters—with these repeaters being used toprovide branch in the network. A repeater such as this is now in theform of a diagnosis repeater 3, which, in addition to the repeaterfunction for the bus segments B1, B2, has a measurement circuit 4 forreflection measurement.

[0029] In the exemplary embodiment, the diagnosis repeater 3 has twomeasurement circuits or devices 4 with terminating impedances (which arenot shown). The measurement devices 4 are connected to the bus line 2and are each associated with one bus segment B2 or B3. Each bus segmenthas a number of bus subscribers Tn which are connected to the respectivebus line 2, with only the bus segment B2 with bus subscribers T11 to T13being used in the exemplary embodiment. A terminating impedance 5 isconnected to the bus subscriber T13. The bus segment B2 has ameasurement initiator 6 as an active bus subscriber T12, whereas thediagnosis repeater 3 is a passive bus subscriber T10.

[0030] The topology of the bus system 1 is determined by a reflectionmeasurement. This is done by making use of the fact that a transmittingbus subscriber Tn on the bus segment B2 or on the bus line 2 behaves inthe same way as a disturbance point F that is symbolized by the arrow.This is because a transmitting bus subscriber Tn reduces its internalimpedance from infinity to about 40 Ω when transmitting. If ameasurement pulse is now applied to the bus line 2, then thismeasurement pulse is reflected by a transmitting bus subscriber Tn.

[0031] A bus subscriber Tn is caused to transmit by the measurementinitiator 6 addressing it with a request signal FS, thus causing it totransmit. In the case of PROFIBUS, a measurement message in the form ofa so-called SRD where DSAP=3FH without data (SRD measurement message) isprovided for this purpose. If the addressed bus subscriber Tn behaves inaccordance with the standard, then it responds with a response signal ASwhich, in the case of PROFIBUS, is a so-called RS (FC=x3H) or RSmeasurement message.

[0032] While a bus subscriber Tn responds to a request signal FS (SRDmeasurement message) with a response signal AS (SR measurement message),the diagnosis repeater 3 transmits a measurement pulse or a measurementsignal MS which is reflected by the transmitting subscriber Tn. Themeasurement device 4 in the diagnosis repeater 3 uses the time periodbetween the transmission of the measurement pulse MS and the arrival ofthe reflection, that is to say of a reflection signal RS from theresponding bus subscriber Tn, to determine the relative position of thisbus subscriber Tn on the bus line 2, and thus within the bus segment B2.In this case, the distance between the respective bus subscriber Tn andthe diagnosis repeater 3 is determined from the delay time divided bytwice the line or cable delay time.

[0033] The diagnosis repeater 3 stores the determined distances ln ofthe bus subscribers Tn in a topology table 7 as illustrated in FIG. 2.The topology table 7 contains all the possible bus subscribers Tn in thebus system 1 (subscriber field TF). The status (status field SF) and theposition on the respective bus segment B2,3 (bus segment field BFT) aswell as the distance ln from the diagnosis repeater 3 (distance fieldEF) are stored for all the bus subscribers Tn in the topology table 7.The status of a bus subscriber Tn which has not yet been measured isopen, while measured bus subscribers Tn have the status “found” or “notfound”. The exemplary embodiment assumes that no bus subscribers Tn areattached to the bus segment B2, and that all the bus subscribers Tn onthe bus segment B2 have been measured. The diagnosis repeater 3, as thebus subscriber T10, is located both on the bus segment B2 and on the bussegment B3, and its distance from itself is L10=0 m.

[0034] If, following the determination of the bus topology, the distanceto a disturbance point, which is symbolized in FIG. 1 by the arrow F onthe bus line 2 or the bus segment B2, is determined, then it is possibleto use the topology table 7 to find out which bus subscribers Tn thisdisturbance point or the line fault F is located between. In theexemplary embodiment, the disturbance point F is located between the bussubscriber T11, whose distance from the diagnosis repeater 3 is l11=5 m,and the bus subscriber T12, that is to say the measurement initiator 6,whose distance from the diagnosis repeater 3 is l12=10 m. The exactdistance IF between the disturbance point F and the diagnosis repeater 2is likewise determined by the measurement device 4 and in this case onceagain preferably on the principle of reflection measurement.

[0035] In order to identify disturbance points or line faults F on thebus line 5, the diagnosis repeater 3 sends an initiating measurementsignal to the bus line 2. This initiating measurement signal isreflected at the disturbance point F as a result of the change of thecharacteristic impedance of the bus line 2, and is detected by themeasurement device 4 as a reflection or reflection signal. The distancelF between the disturbance point or the line fault F and the diagnosisrepeater 3 is determined from the time difference between the start timeof the measurement signal initiated by the diagnosis repeater 3 and thearrival of the reflection signal at the measurement device 4. For thispurpose, the detected delay time is once again divided by twice the lineor cable delay time which is, for example, 5 ns/m.

[0036] The initiating measurement signal is preferably a measurementmessage, that is to say a data message which forms the wanted signal. Apassive reflection measurement which is carried out in this way has themajor advantage over an active reflection measurement that the linediagnosis is free of any reactions, since no separate test signal orseparate measurement pulse need be passed to the bus line 2 duringoperation of the bus. In fact, the actual wanted signal is used todetect line faults or disturbance points F.

[0037] Passive reflection measurement can thus be used not only fortopology determination but also to identify and localize, in a mannerfree of reactions, disturbance points or line faults F, and hence forparticularly effective line diagnosis, even during operation of the bus.

[0038] The integration of the measurement device 4, which is used fortopology determination and preferably also for line diagnosis, in arepeater to whose function the diagnosis repeater 3 has been added inthis way means that no additional or separate bus module is requiredwithin the bus system for this measurement function. The measurementinitiator 6 is an active bus subscriber, expediently a master whichalready exists in such a bus system 1 and to which an appropriateinitiator function has been added. There is thus also no need for aseparate bus module for this purpose.

[0039]FIG. 3 shows a comparatively complex, networked bus system 1 witha number of diagnosis repeaters 3 and with a measurement initiator 6which is once again addressed as a bus subscriber T12. Each of thesediagnosis repeaters 3 in this bus system 1, which in the exemplaryembodiment is in the form of a PROFIBUS DP network, connects a maximumof three bus segments Bn to one another. The diagnosis repeater 3, whichis considered in more detail in the following text and is addressed asthe bus subscriber T11, is also connected via its PG interface to themeasurement circuit 4 of the diagnosis repeater 3, which is addressed asthe bus subscriber T90. Its further measurement circuit 4 is connectedto a programmer (PG), which is addressed as the bus subscriber T91.

[0040] A topology table 7′, which is larger than the topology table 7shown in FIG. 2 and in the exemplary embodiment is associated with thediagnosis repeater 3 that is addressed as the bus subscriber T11, has(in addition to the subscriber field TF and to the status field SF aswell as in addition to the subscriber-specific bus segment field BFT andto the distance field EF) a type field TF for identifying the type ofbus subscriber Tn, and a repeater-specific bus segment field BFR foridentifying a segment-specific adjacent diagnosis repeater 3.

[0041] The entries in this enlarged topology table 7′ were produced as aresult of the automatic determination of the network topology by thediagnosis repeater 3, which is addressed as the bus subscriber T11 andis referred to in the following text as the diagnosis repeater T11. Onthe basis of a low-priority SRD measurement message (SRD at DSAP=63),which is transmitted by the measurement initiator 6 (which is addressedas the bus subscriber T12), to which the bus subscriber T3 which isconnected to the bus segment B1 of the diagnosis repeater T11 respondswith a data message DT ( FIG. 6), the diagnosis repeater T11 hasidentified this as such and labeled it appropriately in the type fieldTF. An entry in the repeater-specific bus segment field BFR of thetopology table 7′ furthermore identifies the corresponding bus segmentB2, via which the diagnosis repeater 3, which is addressed as the bussubscriber T3 and is also referred to in the following text as thediagnosis repeater T3, is connected to the diagnosis repeater T11.

[0042] The status “not found” is stored for this bus subscriber T3 inthe enlarged topology table 7′, since the diagnosis repeater T11 doesnot have a measurement circuit 4 on this bus segment B1. The diagnosisrepeater T11 can therefore not determine the distance to the diagnosisrepeater T3. This distance In is determined by the diagnosis repeaterT3, since this has a measurement circuit 4 on this bus segment B1 or B2.The distance ln between these two diagnosis repeaters 3 is thus includedin the enlarged topology table for the diagnosis repeater T3.

[0043] The low-priority SRD measurement message which is sent by themeasurement initiator 6 results in the diagnosis repeater T11 beingassociated with the distance L11=0 m with itself, as well as the bussegments B1,2,3 which are connected through it and the PG interface andthe status “found”. In an analogous manner, the diagnosis repeaters 3which are connected to the bus segments B2 and B3 of the diagnosisrepeater T11 and are respectively addressed as the bus subscribers T22and T24 lead to corresponding entries in the enlarged topology table 7′of the diagnosis repeater T11.

[0044] The two further bus subscribers T21 and T23 which are connectedto the bus segment B2 of this diagnosis repeater T11 respond to the SRDmeasurement message which is sent by the measurement initiator 6 with anRS measurement message and are, in consequence, identified as bussubscribers Tn, which are not diagnosis repeaters 3. This is reflectedin the type field TF of the respective topology table 7′. The diagnosisrepeater 3 which is connected to the PG interface and is addressed asthe bus subscriber T90 is admittedly once again identified as such, butis likewise given the status “not found”, since the diagnosis repeaterT11 once again has no measurement circuit 4 for it.

[0045] While the bus subscribers T21 to T23 and T24 which are connectedto the bus segments B2 and B3 respond to the low-priority SRDmeasurement message which is transmitted by the measurement initiator 6with an RS measurement message, the diagnosis repeater T11 once againsends a measurement signal MS to the bus subscribers T21 to T24, atwhich point they once again respond with a reflection signal RS. Therespective measurement circuit 4 of the diagnosis repeater T11 onceagain uses the signal delay time between the transmission of themeasurement signal MS and the arrival of the reflection signal RS todetermine the distance L21 to L24 to the corresponding bus subscriberT21 to T24, and enters the result in the appropriate distance field EFin its topology table 7′.

[0046] While the other bus subscribers Tn respond to the low-prioritySRD measurement message of the measurement initiator 6 with an RSmeasurement message, the or each diagnosis repeater 3 responds to itwith a data message DT in addition to a segment identifier SK (8 bits)as shown in FIG. 5, this data message DT contains, as shown in FIG. 6,an identification number IN for the diagnosis repeater 3 (16 bits), acascading identifier or cascading depth identifier KK (8 bits) and atopology updating counter TZ (16 bits).

[0047] The bus segment Bn to which the diagnosis repeater 3 has sent thedata message DT can be read on the basis of the segment identifier SKshown in FIG. 5.

[0048] Furthermore, it is possible to read from this segment identifierSK whether a measurement circuit 4 for reflection measurement isconnected to this bus segment Bn. For this purpose, a first segmentfield F1 is set to logic “0” when no measurement circuit 4 is connectedto this bus segment Bn. Otherwise, this segment field F1 is set to logic“1”. A further segment field F2 is reserved, and is set to logic “0”.The respective bus segment Bn is identified in a third segment field F3,with the identifier “01”, “10” or “11” being set in the case of the PGinterface “00” and in the case of the bus segments Bl,2,3.

[0049] The data message DT which is sent on a segment-specific basis bythe diagnosis repeater 3 is illustrated in FIG. 6. This shows that eachdata message DT which is sent on a segment-specific basis initiallycontains the same identification number IN=80H A7H. The segmentidentifier in the data message which is sent via the bus segment B1 isset to SK=01H since, according to the exemplary embodiment shown in FIG.3, a measurement circuit 4 of the diagnosis repeater T3 is located onthis bus segment B1. The segment identifier in the data message which issent via the PG interface is set to SK=00H, while this segmentidentifier in the data message which is sent via the bus segments B2 andB3 is set to SK=10=0AH or SK=11=0BH, respectively. The cascadingidentifier in each data message DT which is sent on a segment-specificbasis is set to KK=00H.

[0050] This cascading identifier KK indicates the number of thosediagnosis repeaters 3 which have passed on the data message DT ofanother diagnosis repeater 3. The diagnosis repeater 3 shown in FIG. 6has transmitted the data message DT and thus uses KK=00H as thecascading depth identifier. Each diagnosis repeater 3 which passes onsuch a data message DT increments the cascading depth identifier, forexample by one. This means that a diagnosis repeater 3 which receivesthis data message DT sets the identifier to, for example, KK=01H beforeit passes on this data message DT. This identifier KK can thus be usedto encrypt a cascading depth of up to 128 diagnosis repeaters 3,although this is preferably limited to a smaller number of, for example,9 or 10 diagnosis repeaters 3.

[0051] If the measurement initiator 6 sends a low-priority SRDmeasurement message via the bus line 2 within the bus system 1 orPROFIBUS DP network illustrated in FIG. 3 then, for example, thediagnosis repeater T3 responds with a corresponding segment-specificdata message DT, which the diagnosis repeater T11 receives via its bussegment B1. The diagnosis repeater T11 passes this data message DT onvia its bus segments B2 and B3, once it has incremented the cascadingdepth identifier KK in this data message DT. The two diagnosis repeaters3 which are addressed as the bus subscribers T22 and T24 then use thiscascading depth identifier KK to identify that this data message DT hasalready been passed on by one diagnosis repeater 3.

[0052] All the diagnosis repeaters 3 thus monitor the message traffic onthe bus line 2, and create their topology tables. To do this, eachdiagnosis repeater 3 sends a measurement [rod] signal MS in anappropriate manner via its measurement circuits 4 and the respective bussegments Bn in order to determine the distance ln to the respective bussubscriber Tn, while the latter responds to the low-priority SRDmeasurement message from the measurement initiator 6 with a data messageDT in the case of a diagnosis repeater 3, or with an RS measurementmessage in the case of another bus subscriber Tn which is not adiagnosis repeater 3. The totality of the topology tables 7′ of all thediagnosis repeaters 3 thus provides the overall network topology of thebus system 1.

[0053] The invention has been described in detail with particularreference to preferred embodiments thereof and examples, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention.

1. A method for determining the network topology of a bus system (1)having a number of bus subscribers (T_(n)), which are arranged onmutually connected bus segments (B_(n)) of a bus line (2), with each bussubscriber (T_(n)) being sent an initiating measurement message (SRD)which is responded to by a diagnosis repeater (3), which is providedwith a measurement device (4) for reflection measurement and connects anumber of bus segments (B_(n)), with a data message (DT) which has asegment identifier (SK), and is responded to by the other bussubscribers (T_(n)) with a response message (AS, RS), with the diagnosisrepeater (3) sending a measurement signal (MS) to the responding bussubscriber (T_(n)) which reflects this signal, and with the distance(l_(n)) between the responding bus subscriber (T_(n)) and the diagnosisrepeater (3) being determined from the time interval between thetransmission of the measurement signal (MS) and the arrival of thereflection signal (RS).
 2. The method as claimed in claim 1, in whichthe diagnosis repeater (3) increments a cascading identifier (KK) of areceived data message (DT) and passes this on on a segment-specificbasis.
 3. The method as claimed in claim 1 or 2, in which the distance(l_(n)) to the responding bus subscriber (T_(n)) is stored in a topologytable (7, 7′) which is associated with the diagnosis repeater (3). 4.The method as claimed in claim 3, in which a segment identifier (BF) forthe responding bus subscriber (T_(n)) is entered in the topology table(7, 7′).
 5. The method as claimed in claim 3 or 4, in which a statusidentifier (SF) for the responding bus subscriber (T_(n)) is entered inthe topology table (7, 7′).
 6. The method as claimed in one of claims 3to 5, in which the fault location of a line fault (F), which is detectedby means of the measurement device (4), between two bus subscribers(T_(n)) is determined using the topology table (7).
 7. An apparatus fordetermining the network topology of a bus system (1) having a number ofbus subscribers (T_(n)) which are arranged on bus segments (B_(n)) of abus line (2), having a number of diagnosis repeaters (3) which connectthe bus segments (B_(n)) to one another, and each of which has asegment-specific measurement device (4) for reflection measurement, andhaving a measurement initiator (6), which is connected to the bus line(2) and sends a measurement message (SRD) to the bus subscribers(T_(n)), to which the or each diagnosis repeater (3) responds with adata message (DT) which has a segment identifier (SK), and every otherbus subscriber (T_(n)) responds with a response message (RS), with thediagnosis repeater (3) identifying a responding bus subscriber (T_(n))and sending a measurement signal (MS) to it, and determining thedistance to the responding bus subscriber (T_(n)) from the time intervalbetween the transmission of the measurement signal (MS) and the arrivalof a reflection signal (RS).
 8. The apparatus as claimed in claim 7, inwhich each diagnosis repeater (3) has a topology table (7, 7′), in whichthe distance (l_(n)) to the or to each bus subscriber (T_(n)) isentered, and which is identified by this associated bus segment (B_(n)).9. The apparatus as claimed in claim 8, in which one or more of thefollowing fields is associated with the addressed bus subscribers(T_(n)) in the topology table (7′) of each diagnosis repeater (2): astatus field (SF) for indicating the identification of the bussubscriber (T_(n)), a subscriber-specific bus segment field (BF_(T)) forassociation of a bus segment (B_(n)) which is connected to the diagnosisrepeater (3), a distance field (EF) for entering the distance (l_(n)) tothe bus subscriber (T_(n)), a type field (TF) for identifying the typeof bus subscriber (T_(n)), and a repeater-specific bus segment field(BF_(R)) for identifying a diagnosis repeater (3) which is adjacent on asegment-specific basis.
 10. The apparatus as claimed in one of claims 7to 9, in which each diagnosis repeater (3) has two measurement devices(4) for reflection measurement, with each bus segment (B_(n)) havingonly one associated measurement device (4) and with at least onemeasurement initiator (6) being arranged on at least one bus segment(B_(n)).